The invention relates to a method for forming channels of high fluid conductivity in formation parts around a borehole penetrating such formation. In particular, the present invention relates to a method for forming channels of high fluid conductivity in relatively soft acid-soluble formations, such as chalk formations.
The majority of the known techniques designed for increasing the productivity of formations surrounding a borehole apply a combination of formation fracturing and acid treatment. A propping agent may optionally be added for further increasing the productivity.
Many such techniques are known. In British Pat. No. 672,789, a method is described wherein a formation is fractured by injecting a viscous fracturing medium which may have a propping agent added thereto. Subsequently, an acid solution may be injected into the fracture to reduce the viscosity of the fracturing medium and to react with the formation.
U.S. Pat. No. 2,689,009 mentions the injection, at fracturing pressure, of a viscous emulsion of a mineral acid and an oily vehicle, which emulsion carries a propping agent, followed by the injection of a plain acid.
U.S. Pat. No. 3,044,549 describes the injection of a mixture of oil, acid and propping agent at fracturing pressure into an oil-containing formation.
U.S. Pat. Nos. 3,285,340 and 3,481,401 both mention fracturing by means of a viscous medium, followed by the passage of a propping agent into the fracture. Subsequently, the fracture is closed and acid is injected through the fracture containing the propping agent. The agent is either deformable and acid-resistant, or permeable and filled with a surfactant. The agent locally protects the fracture wall against attack by the acid, resulting in the creation of pillars in the fracture walls where they are contacted by the propping agent.
U.S. Pat. No. 3,642,068 describes fracture formation by means of a viscous medium followed by the passage of a propping agent into the fracture. The agent is shifted to a remote location in the fracture by means of an acid that etches those parts of the fracture walls that are close to the borehole. Subsequently the fracture is closed.
U.S. Pat. No. 3,842,911 describes the formation of a fracture and the introduction of propping agent into the fracture, followed by a complete closure of the fracture on the propping agent and the injection of acid under conditions at which the fracture remains closed.
Finally, U.S. Pat. No. 3,768,564 mentions the formation of a fracture without using a propping agent, acidizing the open fracture, and continuing said acidization after the fracture has been closed.
However, none of these prior art techniques provide a solution for the problem of increasing the productivity of relatively soft acid-soluble formations such as chalk formations. Propping agents in fractures formed in such soft formations have been found to be relatively ineffective since they become at least partially embedded in the fracture walls after the fracture is allowed to close, thus allowing the fracture to close around the propping agent. In cases where a channel system is etched in such fracture walls by means of an acid, the etched channel system has been found to collapse and fill up with chalk after the fracture is allowed to close.